Borg assimilation was predatory; every species the Borg encountered was assessed to determine whether its biological and/or technological distinctiveness was considered "worthy of being added to the Collective's perfection". If found to be so, the species was set upon and forcibly assimilated; the Collective considered the species' will in the matter "irrelevant". (TNG: "Q Who", "The Best of Both Worlds"; VOY: "Mortal Coil", et al.) As of the 24th century, the only species known to have been considered unworthy of assimilation were the Kazon. (VOY: "Mortal Coil")

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The Borg generally did not assimilate individuals, and instead preferred to target larger groups such as the crews of starships and the populations of planets. When in the presence of a small number of individuals, Borg drones would ignore them altogether. This policy would remain in effect until an individual demonstrated some quality found worthy by the Collective, or posed a threat to Borg activities. For example, when the USS Raven was first detected by a Borg vessel it was disregarded. By the time the Raven was detected a second time, its crew had developed technologies to elude Borg sensors, making them a more worthy target. (VOY: "Dark Frontier")

When engaging a starship, a Borg vessel presumed tactical superiority and locked onto it with a tractor beam. With the target immobilized, the Borg defeated its shields and either extracted sections of the ship with a cutting beam, transported drones to key areas, or both. (TNG: "Q Who")

With the crew of a vessel assimilated or otherwise subdued, the Borg stripped the ship down for parts to incorporate into their own technology. If possible the assimilated ship was towed into a hangar aboard the attacking Borg ship. (VOY: "Dark Frontier", "Collective") It was rare for the Borg to keep an assimilated vessel intact, except in instances where a Borg crew was forced to abandon their own ship and commandeer their victims'. (Star Trek: First Contact; VOY: "Scorpion, Part II"; ENT: "Regeneration")

Several episodes of Star Trek: Voyager suggested that the Borg cube at Wolf 359 assimilated Starfleet personnel during the battle, and that these new drones were not aboard the cube during its eventual destruction.

"I am Locutus of Borg. Resistance is futile. Your life as it has been is over. From this time forward, you will service us." (TNG: "The Best of Both Worlds")

"We are the Borg. Lower your shields and surrender your ships. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile." (Star Trek: First Contact)

"We are the Borg. Existence, as you know it, is over. We will add your biological and technological distinctiveness to our own. Resistance is futile." (VOY: "Scorpion")

Stage 7: Exo-plating is added, usually directly over-top of the victim's clothing

Stage 8: The victim is linked to the hive mind and a new drone is born

Little is known about the Borg assimilation technique prior to 2368. Observations since that time demonstrated that the Borg assimilated an individual by projecting a pair of assimilation tubules into the victim's neck, injecting nanoprobes into the bloodstream that began connecting the new drone to the hive mind. This was accomplished the moment a Borg drone was within arm's reach of the target. (Star Trek: First Contact; VOY: "Survival Instinct") Only humanoids have been seen as members of the Borg Collective.

The flashbacks in "Survival Instinct" represent the earliest chronological appearance of assimilation tubules, notwithstanding the temporally-displaced Borg in Star Trek: First Contact and "Regeneration". They were not seen being used in the assimilation of Captain Picard or the Hansen family, and it is possible the Borg did not develop this technology (or assimilate it from someone else) until after 2366. The Borg seen in "I Borg" and "Descent" show little interest in assimilation, so there is no way to know if they were equipped with tubules.

During initial stages of assimilation, Borg nanoprobes began attaching themselves to the victim's red blood cells, allowing them to circulate throughout the body. (VOY: "Scorpion") Within minutes, the nanoprobes spread visibly through the capillaries of the victim's skin. The victim, at this point, still retained his or her individuality and had virtually none of the Borg's standard array of defenses. Left unchecked, the nanoprobes soon begin self-replicating and producing larger constructs that form the necessary Borg implants. Among the first major structures assembled was the neural transceiver, allowing the Collective to tap into the victim's mind and usurp control of his or her body. By this time, the new drone's skin coloration had changed to a pale gray and some small implants began to emerge inside and outside the body. (Star Trek: First Contact; ENT: "Regeneration") The drone was then taken to a Borg facility to have larger implants installed, including tools and exoplating. (VOY: "Unimatrix Zero")

Methods for preventing or overcoming Borg assimilation are of paramount importance to cultures that refuse to abandon members of their society captured by the Collective.

In 2153, Phlox determined that exposing himself to omicron radiation could disable the nanoprobes injected into his body by drones using technology originating in 2373. This method was uniquely applicable to Phlox due to his Denobulan physiology, which made him unusually resistant to assimilation. (ENT: "Regeneration")

The most basic, and arguably most practical remedy was to euthanize the intended victim either prior to or in the early minutes of assimilation, before the nanoprobes could install standard Borg defenses. During the Borg's raid of the USS Enterprise-E, CaptainPicard saw this approach to be the lesser of two evils, believing his assimilated crewmen were irrecoverable under the circumstances and better off dying as individuals than living as drones. (Star Trek: First Contact)

When rescuing Picard himself in 2367, the crew of the USS Enterprise-D were not under direct attack by the Borg and therefore had the luxury of considering other options. Upon connecting his neural net to Picard's Borg implants, Lt. CommanderData was able to isolate Picard's individual mind from the Collective. Disconnecting Picard from his implants was considered a risky procedure, but upon the destruction of the cube that assimilated him the Collective lost its hold on him. Physical recovery was swift, although the psychological trauma was more severe. (TNG: "The Best of Both Worlds, Part II", "Family")

In 2368 a Borg cube was disabled in an electrokinetic storm in the Nekrit Expanse, causing approximately 80,000 drones to be disconnected from the hive mind. These former drones settled on a nearby planet, where their limited medical resources allowed them to remove most, but not all, of their cybernetic implants. After the population descended into war, they elected to form a new hive mind as the Borg Cooperative. (VOY: "Unity")

The crew of the USS Voyager was forced to sever Seven of Nine's link to the Collective when, through her, the Borg reneged on its alliance with the starship. Using a neural transceiver installed in him by the Borg Cooperative and a Borg alcove installed aboard Voyager by the Collective, Chakotay tapped into Seven's consciousness in order to distract her while B'Elanna Torres shorted out the console she was attempting to assimilate. (VOY: "Scorpion, Part II")

The Brunali genetically engineered Icheb with a pathogen that would infect the Collective after they sent him to be assimilated. After this pathogen killed all of the adult Borg aboard the cube which assimilated Icheb, the ship's maturation chambers prematurely released the juvenile drones who were not yet fully connected to the Collective, giving Voyager an opportunity to liberate them in 2376. (VOY: "Collective")

A neural suppressant developed by Voyager's EMH in 2377 made it possible for Kathryn Janeway, Tuvok, and B'Elanna Torres to be physically assimilated without being connected to the hive mind and losing their individuality. This allowed them to surreptitiously operate within a Borg cube and sabotage its central plexus. Following their mission the three officers were returned to Voyager where their implants were safely removed without medical complications. Use of the neural suppressant apparently correlated to the lack of psychological trauma suffered in comparison to Picard's experience. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")

Following Voyager's sabotage of the central plexus, the Borg drones who had been connected to Unimatrix Zero were able to regain their individuality and mount a resistance against the Collective. They were not, at least initially, able to remove their Borg implants due to the urgency of the conflict. (VOY: "Unimatrix Zero, Part II")

Successful liberation of an assimilated drone depended largely on the relation between the period of time spent as a drone versus time spent as an individual. Adults and children who were assimilated for only a short time were often easily reintegrated back into their old lives following disconnection from the hive mind. (TNG: "The Best of Both Worlds"; VOY: "Unimatrix Zero, Part II") However, drones who had spent the majority of their lives within the Collective resisted the very notion that they were better off as individuals, and could retain an admiration for aspects of Borg philosophy. Such drones could feel small or lonely without the "voices" of the entire Collective in their thoughts. (TNG: "I Borg"; VOY: "Scorpion, Part II")

In either case, the psychological impact of recovering one's individuality – and thus, the ability to feel the trauma of assimilation and the guilt of one's association with the Borg's atrocities – could be monumental. (TNG: "Family"; VOY: "Survival Instinct")

From a medical standpoint, there were many risks in leaving the Borg Collective, primarily when the implants ceased to suppress the immune system and the body's organs began rejecting implants that have been given sole dominion over vital functions. The extraction of these implants could be challenging even for a skilled surgeon. One known type of complication occurred when deactivated microscopic implants become lodged against one of the cranial nerves. (VOY: "The Gift") Even after the successful removal of the cybernetics, the liberated drone might not have had access to medical technology that was sufficiently advanced to replace whatever body parts were amputated during assimilation. (VOY: "Unity", "Survival Instinct") Certain implants, such as the cortical node, ocular implant, and assimilation tubules, could not be safely removed by Federation medicine as of 2378. (VOY: "The Gift", "Imperfection")